The present invention relates generally to circuits for supplying drive current to lasers or other optical sources, and more particularly to bias control circuits for use in an output stage of a laser driver or other optical source driver.
Laser diodes and other types of semiconductor lasers are in widespread use as optical sources in high-speed optical data transmission applications. Laser diodes are particularly desirable in such applications due to their high optical output power and spectral purity. A laser driver circuit, also referred to herein as simply a xe2x80x9cdriver,xe2x80x9d is used to supply appropriate drive current to a semiconductor laser, so as to control the optical output signal between an xe2x80x9conxe2x80x9d state corresponding to a logic one level and an xe2x80x9coffxe2x80x9d state corresponding to a logic zero level, in accordance with the data to be transmitted.
Conventional semiconductor laser driver circuits are described in U.S. Pat. No. 5,883,910, issued Mar. 16, 1999 in the name of inventor G. N. Link and entitled xe2x80x9cHigh Speed Semiconductor Laser Driver Circuits,xe2x80x9d which is incorporated by reference herein.
In order to optimize the performance of a semiconductor laser based optical system, it is important to precisely control the drive to the semiconductor laser. More particularly, it is generally desirable to minimize the overshoot in the drive signal when the semiconductor laser is driven from the off state to the on state. Significant overshoot can cause automatic gain control (AGC) circuitry in a receiver of the optical system to implement a substantial reduction in the receiver gain, thereby unnecessarily attenuating the desired signal. This will degrade signal-to-noise performance and increase bit error rate, while reducing the distance over which the optical signal can be transmitted at a given quality level. Typically, a maximum acceptable driver output overshoot is no more than about 10% of the average peak-to-peak output signal amplitude.
A significant problem with conventional semiconductor laser driver circuits such as those described in the above-cited U.S. Pat. No. 5,883,910 is that such circuits can cause a driver output signal to exhibit an overshoot which may exceed the above-noted maximum acceptable levels.
A need therefore exists for improved driver circuits, for use with semiconductor lasers and other optical sources, which are configured to limit the overshoot of the driver output signal to acceptable levels.
The invention provides improved optical source driver circuits which meet the above-noted need.
In accordance with one aspect of the invention, a driver circuit for a laser diode or other optical source includes a differential circuit having first and second inputs for receiving differential input data, a current generator circuit for generating modulation current for the optical source in response to the input data, and a variable bias circuit for applying a variable bias to the differential circuit. The current generator is preferably adapted to establish the modulation current for application to one of a first output and a second output of the driver circuit in accordance with the differential data applied to the first and second inputs of the differential circuit. The variable bias circuit may be configured such that the variable bias current generated thereby for application to the differential circuit is a function of the modulation current, thereby controlling an output overshoot of the driver circuit.
Advantageously, the invention can significantly reduce the output overshoot of an optical source driver circuit, e.g., to a level below the previously-mentioned 10% maximum acceptable level, without adversely impacting other performance parameters of the circuit.